1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a gate electrode of a semiconductor device which improves thermal stability of a tungsten/polysilicon structure.
2. Background of Related Art
Generally, in order to reduce gate resistance in a process for forming a gate electrode of a semiconductor device, tungsten(W) having specific resistance order lower than WSix is deposited on a polysilicon so that a gate electrode is formed. When tungsten is reacted with silicon at a temperature of 600xc2x0 C. or greater, a silicide is formed. Accordingly, WNx is formed as a diffusion barrier layer between tungsten and silicon to form a gate electrode having W/WNx/polysilicon structure.
A related art method for forming a gate electrode of a semiconductor device will be described with reference to the accompanying drawings.
FIGS. 1a to 1d are sectional views of process steps showing a related art method for forming a gate electrode of a semiconductor device.
As shown in FIG. 1a, field oxide films 12 are formed in a semiconductor substrate 11 at predetermined intervals, and then the field oxide films 12 are divided into an isolation region and an active region.
A first insulating film 13 for a gate oxide film is formed on the active region at a thickness of about 40 xc3x85 by thermal oxidation method.
As shown in FIG. 1b, a polysilicon layer 14 is formed on an entire surface of the semiconductor substrate 11 at a thickness of about 1000 xc3x85 by low pressure chemical vapor deposition (LPCVD). N+ ions or P+ ions are then implanted into the polysilicon layer 14. When the N+ ions or P+ ions are implanted into the polysilicon layer 14, the polysilicon layer 14 is masked by a photoresist according to devices to be formed, so that the ions are implanted into a specific desired portion of the polysilicon layer 14.
Subsequently, the polysilicon layer 14 is annealed for ten minutes at a temperature of 800xc2x0 C. so that the implanted impurity ions (N+ or P+) are activated.
As shown in FIG. 1c, the semiconductor substrate 11 is subsequently washed by an HF solution and then a WNx layer 15 is formed at a thickness of about 50 xc3x85. A tungsten layer 16 is formed on the WNx layer 15 at a thickness of about 400 xc3x85 and a second insulating film 17 is formed on the tungsten layer 16 at a thickness of about 2000 xc3x85.
Here, the WNx 15 is used as a diffusion barrier between the 153 tungsten layer 16 and the polysilicon layer 14. WNx and TiN are generally used as the diffusion barrier layer. At present, WNx is more frequently used as the diffusion barrier layer. This is because the grain size of tungsten is remarkably reduced, thereby increasing the resistance of pure tungsten two times or more than a W/Si structure in a case where tungsten is deposited on TiN by a sputtering method. This is also because TiN is oxidized during selective oxidation of silicon.
As shown in FIG. 1d, a photoresist (not shown) is deposited on the second insulating film 17 and then patterned by an exposure and developing processes to define a gate electrode region. The second insulating film 17, the tungsten layer 16, the WNx layer 15, the polysilicon layer 14 and the first insulating film 13 are selectively removed using the patterned photoresist as a mask to form a gate electrode 18.
Subsequently, the sides of the gate electrode 18 are selectively oxidized to form a third insulating film on the entire surface including the gate electrode 18. The third insulating film is then etched back to form insulating film sidewalls 19 at both sides of the gate electrode 18.
However, the related art method for fabricating a semiconductor device has several problems.
The diffusion barrier layer, WNx decomposes into W and N2 at a temperature of 800xc2x0 C. or greater. Thus, a silicide may be formed at the boundary between the WNx and polysilicon 14. In this case, WNx fails to act as a diffusion barrier at a temperature of 800xc2x0 C. or greater, thereby reducing thermal stability of the structure in a high temperature process.
Furthermore, if WNx contains more than 10% nitrogen, the WNx is decomposed into W and N2, thereby forming pores in a grain boundary. As a result, when etching a gate, polysilicon is locally over-etched. This may degrade characteristics of the device.
Accordingly, the present invention is directed to a method for forming a gate electrode of a semiconductor device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for forming a gate electrode of a semiconductor device in which a tungsten/polysilicon structure having excellent thermal stability and no pores is obtained.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for forming a gate electrode of a semiconductor device according to the present invention includes: forming a first insulating film, a polysilicon layer and a tungsten layer on a semiconductor substrate; adding oxygen to the tungsten layer; forming a second insulating film on the tungsten layer to which oxygen is added; and selectively removing the second insulating film, the tungsten layer, the polysilicon layer and the first insulating film to form a gate electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.